The best alternatives to SSLreminder are Crawly, SSL Alerts, and Authbase. If these 3 options don't work for you, we've listed a few more alternatives below.
Crawly is a tool that crawls your web applications to find out broken pages to help you to maintain it. Crawly has a powerful alerting system that if any page of your project is broken, you will get a notification. Crawly also offers you to monitor your all pages of your application in historically.
SSL Alerts is a free service to send SSL certificate expiration alerts. It will send you an alert once the expiration is in less than 14 days and daily when there are less than 10 days left.
Setup a fully functioning domain + SSL management service for your SaaS in minutes. Easily enable SSL certificates for all branded domains pointing to your service.
Monitor your SSL certificates on *every* host in your domains for free. Get notifications prior to certificate expirations, and never experience an outage again.
Never forget to renew your SSL certificates. Get notified when a domain is about to expire and receive a weekly summary of all your domains. Add up to 10 domains for free and up to 100 domains with a premium account for only €20,- per year.